This invention relates to a method and means for fault testing repeaters in a transmission system, and more particularly to improvements therein.
PCM transmission systems usually comprise, between stations, a line pair which is connected between repeaters, there usually being one repeater at every mile. While there are two line pairs provided for transmission in both directions, for economy's sake, one cable may contain line pairs for both directions. Each repeater has a receive and a transmit section which is connected in a manner to respond to incoming signals and transmit them in a direction intended for transmission. These repeaters are well known in the art.
In the event of a fault at one of the repeaters, the problem arises as to how one can determine which one of the many repeaters between stations is the defective one. In order to assist in fault detection, an audio frequency filter is provided at each repeater housing, which is coupled to the repeater in that housing. The audio frequency filter passes a specific frequency, which is different for each repeater housing. The output of the audio frequency filter is coupled to a cable pair, otherwise designated as a fault locating pair. By driving the span or system between two stations with special signals at different audio frequency rates and monitoring the signals returned on the fault locating pair, one can identify the repeater housing (hence the repeater) which is at fault.
It was indicated as economically desirable to arrange spans for one cable operation in which both transmit and receive directions use the same cable and share fault locating filters and the fault locating cable pair. With such an arrangement, the fault locating signal may only exist in one direction at a time or any measurements made would be meaningless. This makes it impossible to test the repeaters in the transmit and receive directions or in both directions, from one end by sending test signals from one end down to the opposite end and then looping them so that they return to the original station. With the presently known techniques for testing, assuming a fault did occur with such an arrangement, it would be impossible to detect whether a fault occurred since the received signal could result from a pulse traveling away from the testing end or toward the testing end.
One attempt at solving this problem is to install two amplifiers at each repeater housing and simplex their power on the fault locating pair, through diodes. By reversing the power feed polarity, only one amplifier is enabled and therefore only one direction is tested at a time. However, not only is this arrangement expensive, since two amplifiers and associated circuitry must be installed at each repeater, but it also weakens the maintenance system. The reason for this is that the amplifiers, which are active devices, are subject to the same internal or external damage as the repeaters themselves. For example, a lightning storm is as likely to damage an amplifier as a repeater. It may be also noted that a substantial number of systems are installed without provision for amplifiers and therefore retrofit would be costly. Accordingly, such systems are usually tested first from the office at one end, and then from the office at the other end of the span.